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pychroot

pychroot is a python library and cli tool that simplify chroot handling.
Specifically, the library provides a Chroot context manager that enables
more pythonic methods for running code in chroots while the pychroot
utility works much like an extended chroot command in the terminal.

Usage

In its simplest form, the library can be used similar to the following:

from pychroot import Chroot
with Chroot('/path/to/chroot'):
code that will be run
inside the chroot

By default, this will bind mount the host’s /dev, /proc, and /sys filesystems
into the chroot as well as the /etc/resolv.conf file (so DNS resolution works
as expected in the chroot).

A simple chroot equivalent is also installed as pychroot. It can be used in
a similar fashion to chroot; however, it also performs the bind mounts
previously mentioned so the environment is usable. In addition, pychroot
supports specifying custom bind mounts, for example:

pychroot -R /home/user ~/chroot

will recursively bind mount the user’s home directory at the same location
inside the chroot directory in addition to the standard bind mounts. See
pychroot’s help output for more options.

When running on a system with a recent kernel (Linux 3.8 and on) and user
namespaces enabled pychroot can be run by a regular user. Currently
pychroot just maps the current user to root in the chroot environment. This
means that recursively chown-ing the chroot directory to the user running
pychroot should essentially allow that user to act as root in the pychroot
environment.

Implementation details

Namespaces are used by the context manager to isolate the chroot instance from
the host system and to simplify the teardown phase for the environments. By
default, new mount, UTS, IPC, and pid namespaces are used. In addition, if
running as non-root, both user and network namespaces will be enabled as well
so that the chrooting and mounting process will work without elevated
permissions.

One quirk of note is that currently local variables are not propagated back
from the chroot context to the main context due to the usage of separate
processes running the contexts. This means that something similar to the
following won’t work:

In this case, a NameError exception will be raised unless the variable answer
was previously defined. This will probably be fixed to some extent in a future
release.

Requirements

pychroot is quite Linux specific due to the use of namespaces via the
snakeoil library which also require proper kernel support. Specifically, the
following kernel config options are required to be enabled for full namespace
support:

Release Notes

pychroot 0.9.16 (2016-10-31)

Don’t try to generate new version files if they already exist (fixes another
pip install issue).

Drop py3.3 support.

pychroot 0.9.15 (2016-05-29)

Fix new installs using pip.

pychroot 0.9.14 (2016-05-28)

Move to generic scripts and docs framework used by pkgcore.

pychroot 0.9.13 (2015-12-13)

Add –no-mounts option to disable the default mounts for the command line
tool. This makes pychroot act similar to chroot.

Make pychroot pip-installable without requiring wnakeoil to be manually
installed first.

Add lots of additional content to the pychroot utility man page.

pychroot 0.9.12 (2015-08-10)

The main module was renamed from chroot to pychroot mostly for consistency to
match the project name and cli tool installed alongside it.

Add a man page for the pychroot cli tool.

Add user namespace support so you can chroot as a regular user. Note that
this also requires using a network namespace for which we only setup a
loopback interface (if iproute2 is installed in the chroot) so external
network access won’t work by default in this situation.

Add an option to skip changing to the newroot directory after chrooting. This
is similar to the option for chroot(1) but also allows skipping the directory
change when the new root isn’t ‘/’. In other words, you can use a chroot
environment against the host’s rootfs.

Use $SHELL from the environment for the pychroot script to mirror chroot’s
behavior.

Move WithParentSkip, the main parent/child execution splitting context
manager allowing this all to work, to snakeoil.contextlib and rename it
SplitExec. It was moved in order to develop other context managers around it
in snakeoil and elsewhere more easily.

Allow mount propagation from the host mount namespace to the chroot’s but not
vice versa. Previously systems that set the rootfs mount as shared, e.g.
running something like:

mount --make-rshared /

would leak mounts from the chroot mount namespace back into the host’s
namespace. Now the chroot mount namespace is recursively slaved from the
host’s so mount events will propagate down from host to chroot, but not back
up from chroot to host.

Add support for setting the chroot’s host and domain names for all versions
of python. Previously we only supported setting the hostname for py33 and up.
To set the domain name, pass an FQDN instead of a singular hostname. This
also adds a “–hostname” option to the pychroot script that enables the same
support for it.

pychroot 0.9.11 (2015-07-05)

Fix pychroot script when no custom mountpoints as specified.

pychroot 0.9.10 (2015-04-09)

Add support for custom bind mounts to the pychroot script. Now users are able
to do things like:

pychroot -R /home/user ~/chroot

which will recursively bind mount their home directory into the chroot in
addition to the standard set of bind mounts.

Use “source[:dest]” as keys for mountpoints. This enables support for
mounting the same source onto multiple destinations. For example, with
pychroot it’s now possible to run: